Commuter-rail train systems have been in existence for decades in various metropolitan and rural areas throughout the world. Generally, commuter trains are used to transport people from one location to another. As cities and towns become increasingly populous, more people rely on commuter trains as their primary means of transportation. Thus, commuter train efficiency, reliability, and passenger comfort are important issues for those operating and using the trains
Although commuter trains are a popular method of commuting, improvements in this field are needed. For example, the possibility of delays among the trains is high. When such delays occur, commuter trains can be an unpredictable and unreliable means of commuting. Furthermore, conserving energy and infrastructure costs associated with commuter trains are ongoing considerations for those operating and managing the trains. In addition, improvements in passenger comfort are needed to attract more people to use commuter trains.
As is well known, many commuter rail systems use computers to automatically control their trains, rather than having drivers control the trains manually. These automatic train control systems use circuitry that is directly connected to the rails to locate and communicate with the trains. These circuits divide the track length into "fixed blocks," which can be between a hundred and a thousand feet long. Although these "fixed-block" systems can determine whether a train is present in a given block, they cannot determine where within the block the train is located. Such limitation leads to uncertainty in the location of the trains and requires trains to be separated by large distances for safety reasons. In addition, fixed-block systems typically limit the number of selectable train speeds because of limited communication bandwidth. Thus, trains sometimes must travel slower than the civil speed limit because of the limited number of speed commands. Furthermore, the station stops are affected by this limitation as trains slow down in a "stair-step" braking profile, which is characterized by periodic discrete drops in speed.
The limitations associated with fixed-block systems have prompted those in this field to develop a more efficient and reliable control system. Thus, "moving-block" control systems are being developed to provide a more precise method of locating trains, selecting speeds, and the like. These systems also allow trains to run more closely together, while decreasing the time required for a train to traverse a route. In addition, more sophisticated train control techniques can be used with the moving-block system.
These moving-block systems will enable methods for avoiding low voltages at trains and managing interference to improve the efficiency, reliability, and passenger comfort associated with commuter trains. A more detailed discussion regarding low voltages at trains and managing interference is addressed hereinafter.
Under normal conditions and in a majority of cases, the existing power infrastructure of the train system can sufficiently manage the trains during operation. However, in certain situations when there are power shortages resulting from, for example, multiple trains accelerating at a given time, the existing power infrastructure may be inadequate. For example, when several trains are close together and demand power simultaneously, the voltages at the trains can drop sharply. This results because there is only a limited amount of available power. Insufficient power may be due to track geometry and/or an outage at a traction power substation. When train voltage drops occur, train motor performance begins to degrade, and, for certain types of trains, the motors will eventually shut down to avoid damage from excessive current flow. Furthermore, even with those motors that do not shut down, it is inefficient to allow severe voltage drops because low voltages typically result in large power losses to heat in the rails.
When low voltages at trains occur, a conventional response is to add more power infrastructure until the system is sufficiently robust to handle any situation. Moreover, because the train system must continue to operate during outages, additional power capacity can be installed at substations so that the voltages will be maintained at some desired level during such outages. Rather than installing additional power in existing systems, enhanced control systems can instead be designed to regulate the power.
In the past, simple control strategies have been contemplated and/or employed to avoid low voltages at the trains. For example, one control strategy is to reduce train acceleration rates and top speeds to some arbitrary values. However, this technique has been shown to be ineffective for preventing low train voltages.
Another strategy to avoid low voltages is to use on-board control logic. The on-board controller could reduce power demand as the voltage drops, with power approaching zero as the voltage approaches the desired minimum. Further, the on-board controller could react quickly to a low voltage condition by reducing the power demand of a train in response to the train's measured voltage.
Alternatively, a wayside control approach, where train commands are generated by a computer in a fixed wayside location and then communicated to the trains, allows for more flexibility than the on-board approach. The wayside controller can take into account the train schedule and prioritize the allocation of power among the trains. For example, if two trains traveling in opposite directions are both accelerating, and there is only sufficient power available for one of them, then the on-board control solution would be to cut the power demand of each train in half. However, if one train is on time, and the other is behind schedule, then it may be desirable to allocate most of the power to the high priority train and allow the other to coast. A wayside-controlled algorithm allows such decisions to be made on a case-by-case basis.
An algorithm that prevents the voltage at each train from dropping below some minimum value by limiting power consumption can save energy infrastructure costs, in addition to reducing energy consumption. One objective of the present invention is to apply a control strategy that will allow rush hour traffic to continue operating on the system during a substation power outage with the minimum required power infrastructure. This can be achieved by slowing down trains as needed to keep the system operating smoothly with the available power. The payoff can be substantial because such a control strategy may be sufficient to delay the need for traction power system upgrades, which typically cost tens of millions of dollars.
In order for a wayside controller to maintain the voltage of all trains above some threshold, it must predict train voltages based upon the trajectories of all nearby trains. It may then allocate the available power in such a way as to maintain the voltage at all trains, while minimizing the impact on the schedule. Power consumption can rise quickly enough to take the voltage from a comfortable range to well below the desired minimum in a matter of seconds. Thus, it is insufficient to measure or calculate train voltages and react accordingly, but rather, potential low voltage problems must be recognized before they materialize. The present invention provides a method for recognizing and preventing such voltage problems before they materialize.
Managing interference is also very important to improve commuter train efficiency, reliability, and comfort. "Interference" occurs when a following train travels closely to a lead train such that the following train is forced to brake to maintain a safe following distance. Like cars on a densely packed highway, trains can accelerate and brake repeatedly in response to each other's movements, wasting energy, abusing the motors, and causing an uncomfortable ride for the passengers. Moving-block control systems will allow trains to run close to the minimum safe following distance, so any slight change to a train's trajectory or station dwell time may lead to interference. Removing unnecessary acceleration cycles with enhanced controls will have beneficial impacts on system reliability and energy costs.
As is well known, fixed-block control systems can also exhibit interference behavior. However, the severity of such behavior is moderated by the infrequent changes in train speed commands. In fixed block systems, trains are only given new speed commands when they or the train they are following cross from one fixed block into another. Moving-block control systems, on the other hand, will be capable of changing train speed commands at least once every second, and therefore could exhibit much more severe interference events.
Thus, in the present invention, different types of interference problems are addressed: "Interference During Acceleration"; "Interference Near Station Stops"; and "Interference During Delay Recovery." Managing interference under these conditions can improve passenger comfort, system efficiency and reliability without significantly increasing trip time. The present invention provides methods for dealing with different interference conditions.